Process of molding zirconia



Patented sepe 11,1951

UNITED' STATES PATENT OFFICE 2,567,592

mums 11. Ballard, Niagara Falls, 01mm,

Canada, assignor to Norton Company, Worcester, Mass., a corporation of Massachusetts No Drawing. Application December 19, 1950,

serial No. 201.701

The invention relates to the molding of zirconia otherwise called zirconium oxide of the stabilized variety. This application is a continuation in part of my copending application Serial No. 728,198, filed February 12, 1947, now Patent No. 2,538,959.

One of the objects of the invention is to provide No. 2,125, 88 with little or no reduction. Another object is to improve the process of making bodies from zirconia according to U. S. patent to Ridgway No. 2,091,569. Another object is to increase production of articles molded from zirconia. Another object is to improve the quality of pieces molded from zirconia. Another object is to produce pieces of highdlmensional accuracy. Other objects are to produce articles of stabilized zirconia of excellent quality and by means of a facile process.

other objects will be in part obvious or in part pointed out hereinafter.

According to the invention I provide a mixture oi powders of zirconia, Zl'Oa and lime. CaO.

Both materials are preferably relatively pure but there is a variety of zirconia containing about 4%, more or less, of silica, and this material can be readily molded in accordance with the present invention. Hafnia is chemically indistinguishable from zirconia and is reported as zirconia. Ma-

' terials other than zirconia, hafnia, silica and lime should not be present to the extent of more than 2% of the mixture. In case the zirconia. contains silica, this may be present to the extent of up to 5% of the ZrOa. The amount of lime should be from one percent, which is the least amount that can be considered significant, up to six percent of the amount of ZrO: in the powders, but to produce articles of well stabilized zirconia, lime should be provided to the extent of at least three a percent of the amount of ZIO: in the powders.

Each of the zirconla and the lime should be calcined material of line crystalline structure without recrystallization. This physical condition is sometimes referred to as micro-crystalline and used to be referred to as "amorphous," although that word is not now used for such calcined materials since they are recognized to have a micro-crystalline structure. Thus I use calcined zirconia and calcined lime which have 1 neither of them been fused or heat treated at temperatures high enough to producerecrystallization. The zirconia powder should have been calcined at temperatures not higher than 1200 C. The lime powder should have been calcined at temperatures not higher than 1000 C. ever in each case the temperature of calcination should be high enough to drive oflf nearly all water; water of crystallization, or products of decomposition, such as carbon dioxide.

The following is a table of typical calcining chemical reactions which will produce material suitable for use in this invention:

Table I Oxide calcining Reaction 1000" c. Zirconia Zr(0H)r- Zr0ri-2Hg0.

Lime CaC01oba0+O0g.

The process of this invention involves recrystallization under pressure in contra distinction to what is commonly known as vitrification which involves a fiuxing, or melting of one or more of the constituent oxides. In the indicated amounts the lime goes into solid solution in the zirconia crystals.

I provide a refractory mold, including one or more mold plunger-s. Preferably-the mold is made of graphite which, is a most satisfactory material because it is refractory and is easily machinable. However other materials can be used such as titanium carbide, TiC, zirconium carbide, ZrC, tungsten carbide, WC or vanadium carbide, VC.

I further provide a pressure-sintering fur-. nace, which may be an induction furnace with pressure plungers or a resistor tube furnace such as disclosed in the aforementioned patent to Ridgway No. 2,125,588. In fact any type of furnace which will. heat the contents of a graphite mold, which furnace is also provided with means to exert pressure on the contents of the mold, may be used.

I have discovered that the objects of the invention can be achieved if the zirconia and the lime are in the form of very fin'e particles, that is the major portion of the material being of 10 microns or less. in particle size, and 98% of the material consisting of particles none of which is larger than 50 microns.

Patent No. 2,125,588 '(hereinai' a If a Ridgway furnace-is used,

'turnedonandeventusllytheair Patent 803,125,508). An important feature of the present invention themoldisplacedinthegraphitefurnace tube 50, the furnace plungers130 and 173, are then placed in position. the electric currentis are openedtoapplypressm'eon 2,091,569 wherein a tempera-tine close to the melting point of the oxide is advised The preferred temperature of moldlng'is about i850"C. and the range of temperatures'which can he used according to this invention ls-irom 500C. to 2200' C. The melting point of pure sirconimzroaisil'mo' C.andthemeltin gpoints of the airconia-lime system. up to 6% of lime with ZrOs. are as follows according to the Journal when articles are molded out ofzirconia and lime of the particiesizes specified herein, at tem peratures as specified above and at stated hereinafter, homogeneous pieces of almos pressure valvesthe mold (see which'are notcuhic wiil he ashereinhefore stated. are farhelow the melting pointsofsirconiaandlimeflOO" C. and2572 e respectively. lfasrnuchasti'fioflimein the amcuntofZrOrispresmtappmuimatelyM'ltof the airconia will recrystallilc inthe cubic system. Icseramountsoflimewillcauselesser percentagesofthesirconiatorecry talliseinflie monoclirrlc, which is the crystal habit of haddeleylte (the des- Baddeieyite or monoclinic xii-coma produced in the electric furnace when made into refractory articles such as bricks and other shapes, spails,

theoretical density are produced. Furthermore 'thesm-facesofthesearticlesarefreefromarw apparent reaction of the zirconia or lime with the graphite of the mold, that is to say there has been no reduction even on a surface layer. Thc articles are not spalled even though they have sharp dihedral polyhedral and/or reentrant angles. and the articles can have ribs and grooves andcanhemadeintomanycomplicatedshapes to high dimensional accuracy, 1. e. as good as the checks,'crazes. or fractures when used in high temperature furnaces.- Thi is because monocliniczirconia inversion at a temperature of about 1250 C. The cubicairconiadoesnotnndergoany phase change or inversion at any temperature up to the melting point. -However, the coeflicient of expansionofcubicsirconiaishigherthanthat of monoclinic slrconia so therefore. for certain products,amixtureofthecnhicandmonoclinic forms is preferred andin generalthere should ,heatleast%cubicslrconiaintheproduct.

However, for other uses, 100% cuhicsirconia is preferred, and on the other hand. for certain purpmaminorpercentofthecuhicsirconia inpredominantlymonnclinicalrconiagivesthe bestresults. OncperoentoflimeontheZrO:

will givethislatter product. Such aproduct containingonlyonepercentoflimeonthem isusefulwherealowtof iswantedandthearticleisheatedtoahlgh' temperaturehuti neveroooledduringitsusemamastronge'r" fullife. Thecuhicsireonla productthanthemonoclinicsiroonia,sothereforetheentirerangehetweenl'kandd'ibof limeontheZrOzhas-co of properties mold. Because of the high density'of these ary ticles they are gas tight under a vacuum of as low as one micron of mercury. The articles are substantially integral pieces of zirconia,ZrO:,-

w-ith lime, C90, in solid solution in the crystals thereof, in multi-crystalline form with suiiicient crystalline growth so that the articles have great strength. They fracture with a conchoidal frac- Therealdensity of m l-5.00% CaO on the ZrO: Ls 5.78.1 have madepieces accordingto the invention, with 5% of lime on the ZrOa, havin'gadensityof 5.60. Thisresult canreadilybe duplicated in The mold pressure is preferably as high as the raphite mold will stand, and in any event, 500 pounds to the square inch or. more. For most shapes a graphite mold can be made that will withstand 2500 pounds per square inch. I prethat are desirable and for different uses different proportions within the range will be found ,to hesuperior. Itisbyusingtheprocess of this invention not only to produce exact shapes accordingto the shapes of the molds, but also to achieve great accuracy.

Itwillthusheseenthatthere beenmjovided by this invention a process for the molding of zirconia in which the various objects hereinbei'ore given together with many thoroughlyv practical advantages are ully achieved.

" As various other embodiments might be made of inbefore set forth is to he interpreted as illustrative and not in a limiting sense.

Iclaim:

The rocessor moldingahus,de ,7 integral ofsirconia and lime which com-V.

throughaphasechangeorf 5 prises providing a refractory mold including a hollow mold body and at least one mold plunger. providinga quantity of powder of zirconia ZX'O: of line crystalline structure which has been obtained by calcination at a temperature not higher than 1200 C. and which has not been recrystallized, providing a powder of lime CaO in the amount oi from 1% to 6% of the ZrOz in the powder of zireonia and which powder oi lime CaO is also 01 tine crystalline structure which has been obtained by calcination at a temperature not higher than 1000 C. and which has not been recrystallized. the major portion lo per square inch.

of both powders being 01' particle size not greater than 12 microns and 98% of both powders being of particles none of which is larger than 50 microns particle size, thoroughly mixing said powders, placing a quantity of the mixture in a mold and said mold in a furnace with pressure apparatus, and heating while pressing to a temperature and pressure which rise to between 1500 C. and 2200 C. and to at least 500 lbs.

ARCI-IIBALD H. BAILARD.

No references cited. 

